1. Field of the Invention
The present invention relates generally to high performance amplified telephones. More specifically, a DSP-enabled system and method for increasing intelligibility of audio on amplified telephones by providing digital audio processing customizable based on characteristics of hearing loss specific to individual end users are disclosed.
2. Description of Related Art
Conventional amplified telephones amplify audio for hearing-impaired users. Such conventional amplified telephones generally provide simple tone control and volume control. FIG. 1 is a block diagram illustrating how audio is processed by an exemplary conventional amplified telephone 100. Audio signals are carried over twisted-pair copper telephone wires 102 to a 2:4 wire low-noise DAA (data access arrangement) 104. The DAA 104 is a hybrid, i.e., a circuit that divides a single transmission channel into two, one for each direction, and/or combines two transmission channels into one. Thus, the DAA 104 divides a single transmission channel received from the twisted-pair copper wires 102 into two channels (transmit and receive) and transmits signals on the receive channel to a selectable or optional filter 106. In addition, the DAA 104 also receives a transmission channel input from a microphone 116 of a handset or headset 114, for example, and combines this with the receive channel in the DAA for transmission over the twisted-pair copper telephone wires 102. It is noted that although physical telephone wires 102 are shown and described herein, any suitable method for telephone signal transmission may be implemented such as wireless cellular transmission.
When activated, the selectable filter 106 filters the received audio in order to apply a frequency tilt. Output from the selectable filter 106 is received by the amplifier or volume control 108 which, when the filter is not activated, provides a nearly flat gain typically over a frequency range of 300 HZ to 3500 Hz. Generally, a maximum of 30 dB of gain over nominal is provided by the volume control 108 due to feedback between the microphone 116 and a speaker 112 of the handset 114. The feedback results in generating a howling noise at the speaker 112 similar to when a microphone is placed too close to a public address (PA) system speaker. Nominal is generally considered to be the level output by the telephone speaker 112 that complies with EIA RS-470B when the volume level set to the minimum.
The volume control 108 outputs to a limiter 110 that limits the output level of the telephone speaker 112. For example, the limiter 110 may limit the output level of the telephone speaker 112 to 124 dBSPL (decibel sound pressure level) of acoustic power as measured in a 2 cc coupler in order to meet the safety requirements of UL1950 3rd Edition. As is evident, conventional amplified telephones that compensate for mild to moderate hearing loss generally use only analog circuitry and thus provide only limited capability for the user to tailor the tone response to more closely match the user's hearing loss.
FIG. 2 is a graph illustrating the effect of the frequency tilt of the selectable filter 106. Lines 120, 122, 124 correspond to the effect of the frequency tilt when the volume control is set at low, medium, and high volume settings, respectively. As shown, the frequency tilt filter 106 applies greater amplification for higher frequency audio as volume control is increased. At minimum or low volume gain, there is little or no difference between filtered and non-filtered audio, resulting in a relatively flat frequency response as shown by line 120. It is noted that the frequency tilt should generally not be applied for users with a relatively flat hearing loss. It is further noted that regardless of whether the filter 106 is activated, the response of the microphone 116 is not affected thereby.
As evident from FIG. 2, the amount of frequency tilt generated by the filter 106, if selected, is dependent upon the volume setting. In other words, tone control using the selectable the tilt filter 106 is integrated into the volume control 108 and thus automatically applies a predetermined amount of high frequency emphasis. Although such integrated tone control is relatively simple for the end user, the integrated tone control does not allow each user to tailor the tone response to more closely match the user's type of hearing loss. Rather, the integrated tone control applies the predetermined amount of high frequency emphasis regardless of the hearing loss characteristics of the individual user.
However, the hearing loss characteristics of individual users vary beyond what is compensated by a mere differentiation between a flat response and a frequency tilt or the use of a tone control. In particular, more seriously hearing-impaired users will generally benefit from better audio processing algorithms that can be tailored to better compensate and more accurately match the individual user's hearing loss.
Thus, it would be desirable to provide an amplified telephone with a better audio processing algorithm that can be tailored to compensate for and more accurately match the individual user's hearing loss. Ideally, the amplified telephone would provide more powerful and intelligent amplification for its hearing-impaired users. This is particularly beneficial because an amplified telephone is the often first “hearing aid” many hearing-impaired people purchase.